Field emitters are useful in various applications such as flat panel displays and vacuum microelectronics. Field emission based flat panel displays have several advantages over other types of flat panel displays, which include low power consumption, high intensity and low projected cost. Current field emitters using micro-fabricated metal tips suffer from complex fabrication process and very low yield, thereby increasing the display cost. Thus, an improved field emitter material and device structure, and a less complex fabrication process is clearly desired. This invention addresses all of these issues.
The present invention can be better appreciated with an understanding of the related physics. In general, the energy of electrons on surface of a metal or semiconductor is lower than electrons at rest in vacuum. In order to emit the electrons from any material to vacuum, energy must be supplied to the electrons inside the material. That is, the metal fails to emit electrons unless the electrons are provided with energy greater than or equal to the electrons at rest in the vacuum. Energy can be provided by numerous means, such as by heat or irradiation with light. When sufficient energy is imparted to the metal, emission occurs and the metal emits electrons. Several types of electron emission phenomena are known. Thermionic emission involves an electrically charged particle emitted by an incandescent substance (as in a vacuum robe or incandescent light bulb). Photoemission releases electrons from a material by means of energy supplied by incidence of radiation, especially light. Secondary emission occurs by bombardment of a substance with charged particles such as electrons or ions. Electron injection involves the emission from one solid to another. Finally, field emission refers to the emission of electrons due to an electric field.
In field emission, electrons under the influence of a strong electric field are injected out of a substance (usually a metal or semiconductor) into a dielectric (usually vacuum). The electrons "tunnel" through a potential barrier instead of escaping "over" it as in thermionic of photo-emission. Field emission was first correctly treated as a quantum mechanical tunneling phenomenon by Fowler and Nordheim (FN). The total emission current j is given by ##EQU1## as calculated from the Sehrodinger equation using the WKB approximation. For electrical fields typically applied, v(y) varies between 0.9 and 1.0, and t is very close to 1.0. Hence, as a rough approximation these functions may be ignored in equation (1), in which case it is evident that a "FN plot" of 1n(j/V.sup.2) vs 1/V should result in a straight line with slope-(6.83.times.10.sup.9).o slashed..sup.3/2 .beta.d and intercept (1.54.times.10.sup.-6).beta..sup.2 /.o slashed.. A more detailed discussion of the physics of field emission can be found in R. J. Noer "Electron Field Emission from Broad Area Electrodes", Appli. Phys., A-28, 1-24 (1982); Cade and Lee, "Vacuum Microelectronics", GEC J. Res. Inc., Marconi Rev., 7(3), 129 (1990); and Cutler and Tsong, Field Emission and Related Topics (1978).
For a typical metal with a phi of 4.5 eV, an electric field on the order of 10.sup.9 V/m is needed to get measurable emission currents. The high electric fields needed for field emission require geometric enhancement of the field at a sharp emission tip, in order that unambiguous field emission can be observed, rather than some dielectric breakdown in the electrode support dielectric materials. The shape of a field emitter effects its emission characteristics. Field emission is most easily obtained from sharply pointed needles or tips. The typical structure of a lithographically defined sharp tip for a cold cathode is made up of small emitter structures 1-2 .mu.m in height, with submicron (&lt;50 nm) emitting tips. These are separated from a 0.5 .mu.m thick metal grid by a layer of silicon dioxide. Results from Stanford Research Institute ("SRI") have shown that 100 .mu.A/tip at a cathode-grid bias of 100-200 V. An overview of vacuum electronics and Spindt type cathodes is found in the November and December, 1989, issues of IEEE Transactions of Electronic Devices. Fabrication of such fine tips, however, normally requires extensive fabrication facilities to finely tailor the emitter into a conical shape. Further, it is difficult to build large area field emitters since the cone size is limited by the lithographic equipment. It is also difficult to perform fine feature lithography on large area substrates as required by flat panel display type applications.
The electron affinity (also called work function) of the electron emitting surface or tip of a field emitter also affects emission characteristics. Electron affinity is the voltage (or energy) required to extract or emit electrons from a surface. The lower the electron affinity, the lower the voltage required to produce a particular amount of emission. If the electron affinity is negative then the surface shall spontaneously emit electrons until stopped by space charge, although the space charge can be overcome by applying a small voltage, e.g. 5 volts. Compared to the 1,000 to 2,000 volts normally required to achieve field emission from tungsten, a widely used field emitter, such small voltages are highly advantageous. There are several materials which exhibit negative electron affinity, but almost all of these materials are alkali metal-based. Alkali metals are very sensitive to atmospheric conditions and tend to decompose when exposed to air or moisture. Additionally, alkali metals have low melting points, typically below 1000.degree. C., which is unsuitable in most applications.
For a full understanding of the prior art related to the present invention, certain attributes of diamond must also be discussed. Recently, it has been experimentally confirmed that the (111) surface of diamond crystal has an electron affinity of -0.7.+-.0.5 electron volts, showing it to possess negative electron affinity. Diamond cold cathodes have been reported by Geis et al. in "Diamond Cold Cathode", IEEE Electron Device Letters, Vol 12, No. 8, August 1991, pp. 456459; and in "Diamond Cold Cathodes", Applications of Diamond Films and Related Materials, Tzeng et al. (Editors), Elsevier Science Publishers B. V., 1991, pp. 309-310. The diamond cold cathodes are formed by fabricating mesa-etched diodes using carbon ion implantation into p-type diamond substrates. Recently, Kordesch et al ("Cold field emission from CVD diamond films observed in emission electron microscopy", 1991) reported that thick (100 .mu.m) chemical vapor deposited polycrystalline diamond films fabricated at high temperatures have been observed to emit electrons with an intensity sufficient to form an image in the accelerating field of an emission microscope without external excitation (&lt;3 MV/m). It is obvious that diamond thin film will be a low electric field cathode material for various applications.